Exploration of the lining of the subarachnoid space in the dog by scanning electron microscopy will be continued, with emphasis on the cranial area. Free cells (macrophages) in the spinal area will be subjected to further investigation, using transmission electro microscopy and light microscopy as well. Related studieson the sconning electron microscopy of the early chick blastoderm will be continued. Tracer studies on the spinal subarachnoid space will be initiated.